The present invention relates to a synchronization detection method of a data transmission apparatus using a digital modulation system and a data transmission apparatus to which the synchronization detection method is applied and more particularly to the technique for improving the accuracy of detection of synchronization of a demodulator in a receiver of a transmission apparatus for transmitting information by means of an orthogonal frequency division multiplex (OFDM) system.
In recent years, as a multiplex system for digital radio communication of mobiles or terrestrial systems, an orthogonal frequency division multiplex (OFDM) system has been getting attention which is characterized to be robust to multi-path fading and ghost.
In the OFDM system, signals obtained by digitally modulating a multiplicity of carrier waves of several tens to several hundreds kinds arranged at intervals of a frequency fs with a symbol frequency fsy (=1/Tsy), that is, OFDM signals (orthogonal frequency division multiplex modulated signals) are used to transmit information codes.
When a transmission signal modulated and transmitted by the OFDM system is received and demodulated on a receiving side, it is first necessary to reproduce or recover synchronization from the received OFDM signal.
For this purpose, the following system has been proposed "A Study of Field Pickup Unit using OFDM Modulation Scheme" by S. Moriyama et al., Institute of Television Engineers of Japan, Technical Report Vol. 19, No. 38, pp. 7-12, August 1995. On a transmission side of this system, there are previously inserted into a starting portion of a frame which is a unit of data transmission processing a null section constituted by a no-signal period and a group of synchronization symbols constituted by a sweep signal or the like having a signal component varying from a maximum frequency to a minimum frequency of a transmission band within a predetermined time. On a receiving side, the null section and the synchronization symbol group are detected to recover synchronization. Further, an example of a specific method of detecting the null section is described in copending U.S. patent application Ser. No. 09/096,454.